This invention relates to electrical resistance heating elements, and more particularly, to molded heating assemblies including heating elements.
Electrical resistance heating elements are available in many forms. A typical construction includes a pair of terminal pins brazed to the ends of a Nixe2x80x94Cr coil, which is then axially disposed through a U-shaped tubular metal sheath. The resistance coil is insulated from the metal sheath by a powdered ceramic material, usually magnesium oxide. While such conventional heating elements have been the workhorse for the heating element industry for decades, there have been some widely-recognized deficiencies. For example, galvanic currents occurring between the metal sheath and any exposed metal surfaces of a hot water tank can create corrosion of the various anodic metal components of the system. The metal sheath of the heating element, which is typically copper or copper alloy, also attracts lime deposits from the water, which can lead to premature failure of the heating element. Additionally, the use of brass fittings and copper tubing has become increasingly more expensive as the price of copper has increased over the years. What""s more, metal tubular elements present limited design capabilities, since their shape can not be significantly altered without losing performance.
As an alternative to metal elements, polymeric heating elements have been designed, such as those disclosed in U.S. Pat. No. 5,586,214. The ""214 patent describes a process of making a polymeric heater in which an inner mold is used having a plurality of threaded grooves for receiving a resistance wire. The assembly is first wound with a wire and thereafter injection molded with an additional coating of thermoplastic material, containing a large amount of ceramic powder for improving the thermal conductivity of the coating.
It has been discovered that injection molding a layer of thermoplastic material loaded with large amounts of ceramic powder can be difficult. The viscous polymeric material often fails to fill the mold details and can leave portions of resistance wire coil exposed. Additionally, there can be insufficient wetting between the over molded thermoplastic coating and the resistance wire, with minimal thermoplastic bonding between the inner mold and the over molded thermoplastic coating. This has led to failure of such elements during thermal cycling, since entrapped air and insufficient bonding create crack initiation sites. Crack initiation sites lead to stress cracks that can lead to shorts in emersion applications. Cracks and entrapped air also limit the heating element""s ability to generate heat homogeneously, which tends to create hot and cold spots along the length of the element.
Efforts have been made to minimize hot and cold spots and insufficient bonding between layers of plastic materials having electrical resistance heaters disposed between their layers. In U.S. Pat. No. 5,389,184, for example, a pair of thermosetting composite structures are bonded together using a heating element containing a resistance heating material embedded within two layers of thermoplastic adhesive material. The two thermosetting components are permitted to cure, and then while applying pressure to the joint, electrical energy is passed through the heating element sufficient to heat the joint to above the melting temperature of the thermoplastic adhesive material. This heat fuses the layers of the thermoplastic adhesive to join the thermosetting materials together. The heating element remains within the joint after bonding and provides a mechanism to reheat the joint and reverse the bonding process in the field. While these procedures have met with some success, there remains a need for a less expensive, and more structurally sound, electrical resistance heating element.
The thermoplastic injection molding process has existed for several years. The plastic molding process has evolved to a point where the standard is high quality detailed, complex shapes, and smooth aesthetic surfaces. In addition, injection molding using plastic part tooling and molding equipment has evolved into a precise science capable of mass producing high quality plastic products.
Typical injection molding processes require that molten plastic be shot into a tool at an extreme high velocity. It is the interaction between the viscosity of the molten plastic, the molding pressure, and the tool geometry that creates a high quality, high detailed plastic part. Another common practice of injection molding incorporates the use of rigid inserts (i.e. insert molding) such as threaded bosses and ancillary mechanical parts. The required material fill velocity and mold pressure, however, are not conducive to accurate placement of element precursors within complex designs. The inability to overcome the adverse effects of mold flow on precursor element placement has limited molded heated part geometries to primarily flat shapes.
Therefore, along with the need for a less expensive, and more structurally sound, electrical resistance heating elements, there remains a need to implement element precursors into molded contoured shapes beyond geometries that are primarily flat surfaces.
The present invention comprises a heated element assembly and method of manufacturing heated element assemblies. The heated element assembly according to the present invention includes a first molded section and a second molded section shaped to mate with the first molded section. The assembly also includes a resistance heating element comprising a supporting substrate having an electrical resistance heating material fastened to the supporting substrate. The electrical resistance heating material forms a predetermined circuit path having a pair of terminal end portions. The first and second molded sections are connected to substantially encompass the circuit path such that the heating element is secured between the first and second molded sections by an interference fit.
The present invention provides several benefits while opening infinite design opportunities. Cost effective complex assembly shapes are easily formed while accurately positioning resistance heating elements. This allows for the ability to provide heat on horizontal planes, vertical planes, and along complex contoured shapes. The supporting substrate is also capable of functioning as a thermal buffer between the resistance heating material and a molded section. Additionally, the supporting substrate serves as a mechanical stress buffer between the resistance heating material and a selected polymer in a heated element assembly. Further, the seam formed between the molded sections allows air to be evacuated from the area formed between the molded sections and a hermetic seal to be formed at the seam, such as by electro-fusing, spin welding, sonic welding, hot air welding, vibration welding, diffusion bonding, or o-ring snap fitting the mated sections together. The heat distribution of a heated element assembly may also be improved by back-filling an inert gas, such as argon, into the area formed between the two mated sections before hermetically sealing the seam.
In another embodiment of the invention, the electrical resistance heating material of the resistance heating element is prevented from contacting the molded sections. The resistance heating element may include a second supporting substrate, and the electrical resistance heating material is fastened between the first and second supporting substrates. Alternatively, the resistance heating element may be suspended between the molded sections along the edges of the supporting substrate. By doing so, the molded sections are buffered by a supporting substrate and/or separation from the resistance heating material, allowing for the transfer of heat to the molded sections without compromising the integrity of the molded sections. This, in turn, permits the use of less heat resilient, but less expensive, polymers to construct the molded sections. Further, different supporting substrates or substrate thicknesses may be selected to bias generated heat to a selected molded sections. This provides for the ability to design and predict heat flow in heated element assemblies.
In still another embodiment of the invention, at least one of the molded sections includes a protrusion extending from a surface facing the resistance heating element. The protrusion contacts the resistance heating element and may extend to contact only the supporting substrate or substrates in order to further locally secure the heating element in a preselected location. Alternatively, the protrusion may contact the electrical resistance material and partially yield to the resistance material to secure the heating element in its selected position.
The invention also provides the ability to selectively control heat distribution through the design of the molded sections. The molded sections may have different thicknesses, be constructed of different materials or include different conductive additives, or a combinations thereof. This ability allows for a design to control the heat transfer and direct generated thermal energy in an application specific manner, such as for cooking, biological processing, or printing applications.
The above and other features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention which is provided in connection with the accompanying drawings.